When a dial-in incoming call is received in a telephone exchange system, the system causes a particular telephone to receive the call by converting a number received from a telephone station into a number used in the system. Therefore, the system holds a table of received numbers of dial-in incoming calls and numbers after conversion, and retrieves the received number and carries out conversion of the received number.
In relation to the above system, Patent Literature 1 discloses a system having an private telephone exchange in which the exchange refers to a dial-in/extension correspondence table when a dial-in incoming call comes, and if there is a registration of a pair of a telephone number of the caller and an external telephone number, the exchange calls an extension telephone of an extension number corresponding to the pair. If there is no registration, the exchange refers to an extension group table and calls all extension telephones which belong to an extension group corresponding to the external telephone number.
Also, Patent Literature 2 discloses a private branch exchange system in which, when a dial-in incoming call comes, the system refers to a dial-in number/extension number connecting thereto correspondence conversion table and converts a received dial-in number into a corresponding extension number. If there is a registration of a number received from the caller in a caller number/extension number correspondence registration table, the system causes a terminal to receive the call, the terminal being connected to a port set to an extension number corresponding to the received caller number. If there is no registration, the system causes a terminal to receive the call, the terminal being connected to a port set to an extension number corresponding to the received dial-in number.